One female patient with cancer pain due to bone metastasis from breast cancer was initially treated with Xinhuang tablets, diclofenac sodium double-release enteric-soluble capsules, paracetamol dihydrocodeine tablets and paracetamol oxycodone tablets, before being switched to controlled-release oxycodone hydrochloride tablets. She regularly took oxycodone hydrochloride sustained-release tablets 20 mg, q12h, no abnormalities were observed, and the dosage was increased to 40 mg, q12h due to poor pain control. The patient was diagnosed with 5-hydroxytryptamine syndrome after 1 d of intermittent recurrent tremor, myotonia, scalp sweating, restlessness and elevated blood pressure. When oxycodone hydrochloride sustained release tablet was adjusted to 20 mg, q12h and gabapentin capsule was added to 0.1 g, tid, the frequency of tremor and myotonia attacks slightly decreased, and sweating and agitation symptoms were not relieved. After 14 days, oxycodone hydrochloride sustained release tablets were stopped and morphine sulfate sustained release tablets 60 mg, q12h were replaced. Three days later, the patient's symptoms disappeared. During 5-month follow-up, the patient's pain was well-contrdled, with no change in the dose of morphine sulfate sustained-release tablets, and no adverse drug reactions observed. Using the Naranjo’s Assessment Scale, the association between the patient's serotonin syndrome and the suspected drug oxycodone hydrochloride sustained-release tablets was evaluated as "probable". This case highlights the importance for clinicians to closely monitor adverse reactions induced by rapid opioid dose escalation to ensure medication safety in patients.

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